This proposal is an application for an electron microscope submitted to the NIGMS shared instrumentation Program. The application requests replacement of an out-dated and poorly functioning RCA-3 electron microscope with a Jeol 100 S electron microscope. The Jeol 100S will be available as a multiuser instrument to members of the core-user group and to the other qualified scientists listed in the proposal. The instrument will be made available to other qualified investigators in the geographical area as the need arises and time permits.